Since a material formed from GaN, AlN, InN, or their mixed-crystals, which is used in a nitride semiconductor, has a wide band gap, the material has been used for a high power electronic device, a short-wavelength light emitting device, or the like. Technology related to an FET (Field Effect Transistor), and in particular, an HEMT (High Electron Mobility Transistor) as a high power electronic device has been developed. The HEMT using the nitride semiconductor is used for a high power high efficiency amplifier, a high power switching device, and the like.
The HEMT used for the above-described applications is desired to be in a normally-off state, and to have a high withstand voltage, and the like. In particular, since the normally-off state is important for safe operation, various methods to achieve this have been considered. As one of the methods for a normally-off HEMT, a technology is presented to form a gate recess by removing a part of a semiconductor layer immediately under a gate electrode. In a recessed-gate structure formed by this method, advantageously, a threshold voltage is retained at positive without increasing a resistance element between electrodes. Also, for a normally-off semiconductor device used for a power application, a high drain breakdown voltage and a high gate breakdown voltage have been desired. Accordingly, in a horizontal structure of the FET and HEMT, a MIS (Metal Insulator Semiconductor) structure forming an insulating film to be a gate insulating film is applied. As described above, a combined structure of the recessed-gate structure and the MIS structure is applied for the HEMT using a semiconductor material related to GaN, in order for the semiconductor device to be suitable for the power application.
However, in the HEMT having the above-described MIS structure, there is a case where the threshold voltage changes in an operation of a transistor due to an interfacial state between a semiconductor and an insulating film which will be the gate insulating film, quality of the insulating film, and the like.
It is considered that when an expected trap level is formed at an interface between the semiconductor and the insulating film, or at the insulating film due to the interfacial state and the like, an electron is trapped by the trap level, and a distribution of electrons is influenced in a 2DEG (Two-Dimensional Electron Gas) immediately under the gate electrode. As described above, when the threshold voltage changes, a current value and an on-resistance are changed when a transistor is operated. As a result, a uniform characteristic cannot be acquired. Also, a yield ratio is decreased.